Chitin is the main component of the cell wall of fungi, plays a key role in the shaping of fungal cells, and is required for maintaining the integrity of the structure of cell wall and the growth and development of the cells. Chitin is a linear chain-like polymer formed by connecting N-acetyl-glucosamine via β-1,4-glycosidic bond and is catalytically synthesized by chitin synthase (Chs). Chitin synthase is a membrane-binding protein, which catalyses the transfer of N-acetyl-D-glucosamine from UDP-N-acetyl-D-glucosamine to a growing chain of chitin for continuous synthesis of chitin. Inactivation of the chitin synthase gene may result in a disorder of the structure of cell wall, deformed fungal cell, unstable osmotic pressure, and an increased sensitivity to the change of osmotic pressure outside (Specht et al., The chsD and chsE genes of Aspergillus nidulans and their roles in chitinsynthesis. Fungal Genet Biol. 1996. 20: 153-167).
The number of the chitin synthase genes varies in different fungi. An ancient fungus, Encephalitozon cuniculi, has only one chitin synthase gene, while Rhizopus oryzae has more than 20 chitin synthase genes (Latgé et al., The cell wall: a carbohydrate armour for the fungal cell. Mol Microbiol. 2007. 66: 279-290). The chitin synthase genes play different functional roles even in the same fungi. For example, the genome of Fusarium oxysporum contains 6 chitin synthase genes (Martín-Urdíroz et al., a class VII chitin synthase involved in septation, is critical for pathogenicity in Fusarium oxysporum. Eukaryot Cell. 2008. 7: 112-121). Inactivation of Chs1 gene resulted in no significant change in the phenotype and pathogenicity of the mutant strain, but resulted in a much higher average number of nucleoli per cell than that of WT, and a reduced content of chitin in the mutant by 10%. Inactivation of Chs2 and Chs7 genes resulted in a decreased pathogenicity, with no significant change to the number of nucleoli per cell. Inactivation of Chs2 gene further resulted in 10% decrease in the level of chitin (Martín-Urdíroz et al., Role of chitinsynthase genes in Fusarium oxysporum. Microbiology. 2004. 150: 3175-3187). No mutant with an inactivated Chs3 gene was obtained, which suggested that the inactivation of this gene has a lethal effect. Inactivation of ChsV gene would result in a dramatic decrease in the pathogenicity of the mutant strain which lost its ability to infect a plant, had severely damaged integrity of cell wall, and was very sensitive to fungicides (Madrid et al, Class V chitinsynthase determines pathogenesis in the vascular wilt fungus Fusarium oxysporum and mediates resistance to plant defense compounds. Mol Microbiol. 2003. 47: 257-266). Inactivation of ChsVb gene resulted in a more significant change in the phenotype of the mutant strain which had an abnormal septum, and balloon-like structures appeared in different parts of hyphae. In the mutant strain, a special phenomenon, called intrahyphal hyphae, could be observed by TEM. The mutant strain totally lost its pathogenicity, and was very sensitive to fungicides. As can be seen, ChsV and ChsVb genes play an important role in maintaining the integrity of the cell wall of Fusarium oxysporum and in infecting plants (Martín-Urdíroz et al., a class VII chitin synthase involved in septation, is critical for pathogenicity in Fusarium oxysporum. Eukaryot Cell. 2008. 7: 112-121). A Chs1 gene was cloned and separated from Fusarium graminearum (Li et al., Cloning and characterization of a gene coding for a class I chitin synthase from Fusarium graminearum. Can. J. Plant Pathol. 2003. 25: 240-248). It was found that a significant change in the structure of the cell wall of a ΔChs1 mutant strain obtained by homologous knockout occurred. The activity of the chitin synthase, the level of chitin, the number of conidia, the length of large conidia, and the pathogenicity for wheat of the ΔChs1 mutant strain significantly decreased as compared to WT (Xu et al., Disruption of the chitin synthase gene Chs1 from Fusarium asiaticum results in an altered structure of cell walls and reduced virulence. Fungal Genet Biol. 2010. 47 (3): 205-215).
As chitin is an important functional component of fungal cell wall, and only specifically appears in fungal cell wall and the shell of crustaceans, insects and other arthropods, but not in plants and mammals, chitin and chitin synthase become ideal targets for developing new fungicides and controlling fungus diseases (Bowman and Free et al., The structure and synthesis of the fungal cell wall. BioEssays. 2006.28: 799-808; Latgé et al., The cell wall: a carbohydrate armour for the fungal cell. Mol Microbiol. 2007. 66: 279-290).
In recent years, the use of host-induced gene silencing (RIGS) technology in controlling plant diseases provides a new approach to the control of plant diseases. RNAi vectors against target genes which are important for the growth, development, and pathogenicity of pathogens were expressed in host plants such that, when the plants were invaded by the pathogens, the expression of the endogenous target genes in the fungi would be interfered with the corresponding dsRNA/siRNA, which resulted in the inhibition of the invasion of the pathogens (Nowara et al., RIGS: host-induced gene silencing in the obligate biotrophic fungal pathogen Blumeria graminis. The Plant Cell. 2010. 22: 3130-3141; Koch et al., Host-induced gene silencing of cytochrome P450 lanosterol C14α-demethylase-encoding genes confers strong resistance to Fusarium species. Proc Natl Acad Sci USA. 2013. 110 (48): 19324-19329). The choice of target genes is crucial for the effect of HIGS. Genes which can be used as the target of RIGS include key genes for growth and development and lethal genes, crucial genes for pathogenicity, and genes required for the invasion of pathogens. Once RNAi vectors constructed with the sequences of these target genes are introduced into a host plant, the expression of the target genes of the pathogens will be silenced by the host through an RNAi pathway, and thus the disease are controlled. With targeting specificity based on nucleotide sequence, this technology enables the generation of stable environment-friendly transgenic plants, and exhibits potential in application.